The present invention relates to a hydraulically operated continuously variable transmission comprising a hydraulic pump and a hydraulic motor, and more particularly to a hydraulically operated continuously variable transmission which includes a hydraulic pump and a hydraulic motor that are hydraulically connected to each other by a closed hydraulic circuit defined in a motor cylinder which includes a tubular portion incorporating an end sealing seal assembly.
Heretofore, continuously variable transmissions comprising a hydraulic pump and a hydraulic motor are known in the art, and employed in various applications. For example, Japanese Patent Publication Nos. 32(1957)-7159 and 56(1981)-50142 disclose a continuously variable transmission having an input shaft to which a fixed-displacement hydraulic pump is connected, an output shaft to which a variable-displacement hydraulic motor is connected, and a closed hydraulic circuit through which oil discharged from the hydraulic pump is delivered to the variable-displacement hydraulic motor for thereby actuating the hydraulic motor to rotate the output shaft.
In such a continuously variable transmission, it is known to provide a seal assembly between a motor cylinder and a fixed shaft as disclosed in Japanese Laid-Open Patent Publication No. 62(1987)-37563. The fixed shaft is inserted in a tubular portion of the motor cylinder, and the interior of the tubular portion is divided into inner and outer spaces by a distribution ring attached to the fixed shaft. These inner and outer spaces provide a closed hydraulic circuit by which the pump and the motor are hydraulically connected to each other. The seal assembly serves to prevent oil from leaking from the closed hydraulic circuit, so that the volumetric efficiency of the pump will not be reduced.
The seal assembly is known as a mechanical seal which comprises a rotatable seal member mounted on the tubular portion, and a fixed seal member mounted on the fixed shaft, the rotatable and fixed seal members having respective end faces held in slidable contact with each other to seal the closed hydraulic circuit. A slight clearance is usually defined between the contacting end faces of the seal members so that the pressure under which the end faces are held in contact with each other can be regulated under the pressure of oil which is to be sealed in the closed hydraulic circuit.
If the clearance were selected to be excessively large, then a large amount of oil would leak through the clearance when the oil pressure in the closed hydraulic circuit is low, resulting in a reduction in the volumetric efficiency. Conversely, if the clearance were selected to be too small, then the contacting end faces would run short of oil film therebetween when the oil pressure in the closed hydraulic circuit is high, with the result that seizure would be caused between the end faces of the seal members.
To avoid the above drawback, the clearance may be dimensioned so that it is of a certain large size, and a spring may be used which can maintain the end faces of the rotatable end fixed seal members in mutually contacting relationship. In order for the spring to preload the end faces under a substantially constant pressure, the spring is required to be of a certain length, and a space for accommodating the spring must be provided.